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    <script>
       /**
        * 如何改变 promise 的状态?
        *   1. resolve              pending =>  fulfilled
        *   2. reject               pending =>  rejected
        *   3. throw 抛出一个错误    pending =>  rejected
        * 
        * 状态只能改变一次.
        */

        let p = new Promise((resolve,reject) => {
            resolve('ok');
            reject('error');
        });

        console.log(p);
    </script>
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